1. Field of the Invention
The present invention relates to a digital watermark embedding apparatus, method and program for embedding digital watermark information in content, such as digitized still image data, moving picture data, voice data, and music data, and a digital watermark detection apparatus, method and program for detecting digital watermark information in content.
2. Description of the Related Art
(Digital Watermarking)
Digital watermarking is a technique for embedding certain information in digital content (digital literary work data), such as digitized still image data, moving picture data, voice data, and music data, by varying the content to a degree in which its quality is not greatly degraded. For example, the identification information of a copyright holder or content user, information concerning the rights of a copyright holder, the conditions for using content, secret information needed to use content, or copy control information, etc. (hereinafter referred to as “watermark information”) is embedded so that it is not easily perceived. When necessary, the thus-embedded watermark information is detected in the content and used for controlling the use of the content or for promoting the secondary use of the content. Further, it has been proposed to use watermark information for such purposes as identification or certification of a copyright holder, fingerprinting, certification of content, and monitoring of broadcasts.
(Requirements for Digital Watermarking)
For watermark information to be used to prevent illegal use of digital content, it is necessary to make the information robust (preventing the content from being lost or being illegally altered) against various operations or attack attempts that may be made on the content.
(Geometrical Distortion)
If the digital content is image data (still images or moving pictures), there is an attack attempt to erase the digital watermark embedded therein by geometrical distortion. Geometrical distortion means coordinate transformation of an image. That is, geometrical distortion changes the positions of pixels. Following a change in pixel position, some types of digital watermark information cannot be correctly detected.
Geometrical distortion is mainly classified into global transformation and local transformation. Global transformation means the scaling, rotation and parallel displacement of the whole image. Global transformation is expressed as an affine transformation. On the other hand, local transformation includes both transformation expressed by a local parameter, and transformation, such as global transformation, expressed by a parameter that does not relate to the position. In other words, global transformation is a special case of local transformation.
(Topological Digital Watermarking)
It is known that homeomorphic spaces have a constant property called “a topological invariant”. Homotopy classes are examples of the amount (see, for example, Iwanami Mathematic Dictionary 3rd Edition, Topological Space, pp. 22-34; Homotopy Theory, pp. 1142-1150).
Digital watermarking may be related to the topological invariant by regarding local geometrical distortion as a homeomorphic mapping. A topological digital watermarking system is a technique for realizing robustness against local geometrical distortion (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-142094). In topological digital watermarking, the digital watermark is a topological invariant (e.g., a homotopy class), which is invariant under local geometrical distortion, and is expressed by a function ψb corresponding to a predetermined homotopy class b. The digital watermark is detected by computing the homotopy class b from the detected function ψ′b.
In conventional digital watermarking techniques, digital watermark information is embedded as a topological invariant into content in a simple manner.